Common carriers now provide all-digital central office facilities for data subscribers. In conjunction with one type of digital facility, the date customer provides a synchronous data bit stream at certain ones of different signaling rates and code formats, the digital facility imbeds this data stream into data bytes which are conveyed with the data of other subscribers through the use of, for example, multiplexers to remote carrier offices and the customer data is there reconverted to the original synchronous data bit stream for transmission to a terminating data terminal or base. A common carrier office facility of this type is shown in Reissue U.S. Pat. No. Re 29,215, issued May 10, 1977 to A. C. Carney et al.
The message data information that the subscriber intends to convey to the terminating data terminal (or data base) may take the form of a burst of message data characters. The initial portion of the data burst customarily includes a control or synchronizing code word having a predetermined bit sequence. This code word indicates that a stream of data characters immediately follow, the code word and following characters forming a data block. This date block is transmitted by the subscriber as a sychronous data stream at the appropriate signaling rate to the common carrier facility of the type described above. The common carrier facility imbeds the code word and data character bits of the customer's data into the "character" bit time slots of the byte train (other bits or time slots of each byte being reserved for housekeeping and control purposes). This is advantageously accomplished by reading successive groups having a fixed number of bits (six in the Carney et al reissue patent) from the synchronous data stream and inserting the bits of each group into time slots of each of successive bytes of the byte train. Since the customer code format customarily provides data characters having a different fixed number of bits (in excess of six) and code words that consist of two characters, it is apparent that each of the message characters will be imbedded into two, and sometimes three, successive bytes in the train, each of the code words will be imbedded in three or more successive bytes and corresponding bits of the several characters will be imbedded in different locations in the several bytes.
The bytes with the imbedded bits are multiplexed with bytes containing data from other customers for transmission to a remote carrier office which demultiplexes the multiplexed bytes and recovers the synchronous stream for transmission to the terminating subscriber. This latter subscriber looks for the synchronizing code word and, upon its detection, recognizes that the first data character in the burst is located immediately thereafter, enabling the reassembling of the characters in the original data burst.
It is sometimes necessary for a remote digital facility to reassemble a data burst from an incoming byte train. This may be necessary when the remote facility is the recipient of the message information, requires addressing information in the burst to properly direct the message or for other similar reasons. The remote facility, of course, may recover the original synchronous stream and, thereafter, look for the synchronizing code word whereupon, upon detection of the code word, the facility may initiate the reassembling of the data characters in the burst. This, however, requires two sequential operations before the assembling operation can take place.
It is an object of this invention to determine the location of bits of data characters imbedded in a data byte train. It is a more specific object of this invention to use fewer and simpler operations to determine the locations of the character bits.